Bridge circuits for measurement purposes typically comprise four impedance elements, at least one of which is a sensor element, e.g., a strain gauge for length measurements or a thermistor for temperature measurements. Such a bridge circuit is advantageous in that extremely small impedance variations of the sensor element can be readily detected. The bridge circuit can be supplied with relatively high voltages and currents, respectively. The output signal corresponding to the measured magnitude, however, is a voltage or current corresponding only to the impedance variation of the sensor element but not to the absolute impedance thereof. Thus, the output signal does not contain the relatively high voltage or current which is impressed across the bridge circuit at the sensor element.
Unfortunately such bridge circuits have no common reference potential between the supply voltage or supply current on the one hand and output voltage or output current on the other hand and thus galvanic isolation is necessary between the power supply and output signal processing circuit.
It is well known that in order to avoid the need for such galvanic isolation the bridge output signal can be coupled to a differential amplifier. The output signal of this differential amplifier is then referred to the common ground of the circuit. However, such differential amplifiers must have high common mode rejection in order to achieve a precise measurement of the differential bridge output signal which is usually small as compared to the supply voltage or supply current. For practically realizing such differential amplifiers precision components are usually required, often requiring pairs of precision resistors to achieve a desired common mode rejection of, for example, 60-80 decibels. However, even precision components are temperature dependent. This temperature dependency can be compensated only with additional expensive circuits or temperature compensated resistors.
British Pat. No. 2,068,130 discloses a bridge circuit for measuring purposes one node of which is drawn to virtual ground by means of an operational amplifier. This bridge circuit is included in an oscillator in which the frequency varies with the variation of the sensor impedance in at least one branch of the bridge. Thus, this circuit is not suitable for applications where a DC output signal is required.